In a conventional technique, a NOx storage/reduction catalyst (referred to hereafter as an NSR catalyst) and a NOx selective reduction catalyst (referred to hereafter as an SCR catalyst) are disposed in an exhaust passage of an internal combustion engine (see Patent Document 1, for example). The NSR catalyst stores NOx contained in exhaust gas that flows into the NSR catalyst when an oxygen concentration of the inflowing exhaust gas is high, and reduces the stored NOx when the oxygen concentration of the inflowing exhaust gas is lowered and a reducing agent is present. The SCR catalyst selectively reduces the NOx using a reducing agent.
Incidentally, the SCR catalyst may be provided on a downstream side of the NSR catalyst, and an amount of reducing agent supplied to the SCR catalyst may be determined in accordance with a NOx concentration of the exhaust gas flowing into the SCR catalyst. In this type of system, a NOx sensor is provided downstream of the NSR catalyst and upstream of the SCR catalyst.
Here, when an air-fuel ratio of the exhaust gas is lowered to or below a stoichiometric air-fuel ratio in order to supply a reducing agent to the NSR catalyst, there are cases where ammonia is formed in the NSR catalyst. This ammonia is detected by the NOx sensor in a similar manner to the NOx. Hence, when the NOx stored in the NSR catalyst is reduced, a detection value of the NOx sensor is larger than an actual NOx concentration. When, at this time, the amount of reducing agent to be supplied to the SCR catalyst is determined on the basis of the detection value of the NOx sensor, an excessive amount of reducing agent may be supplied to the SCR catalyst. As a result, the reducing agent may flow out of the SCR catalyst. Moreover, an amount of consumed reducing agent may increase.